Cellulose acetate tow is a material that is shipped in bales. Tow refers to a continuous band (or bundle) of filaments. Typically, the tow is drawn directly from the bale (i.e., de-baling) for subsequent processing. Therefore, it is important that the tow can be drawn from the bale without difficulty. Moreover, these bales should be stackable which typically means that these bales should have flat or substantially flat surfaces.
In the production of the tow bale, it is necessary to compact (or press) the tow to form the bale. Tow compaction, which is performed in a press, can lead to several problems. One such problem is bales with rounded (i.e., ‘not flat’ or ‘crowned’) tops and bottoms. This is a problem, during storage, handling, and shipping, because the rounded bales can not be stacked easily and have a tendency to tip. Another problem arising during compaction is that the tow can become entangled (i.e., the layers of tow are intermingled). This is a problem, during de-baling of the tow, because the entangled tow may not be easily removed from the bale.
In the recent past, several in the cellulose acetate industry have addressed the problem of rounded bales.
In US Patent Publication No. 2005/0161358, a bale with substantially flat sides (i.e., the top and bottom) is made by a technique in which the compressed tow is packaged in an air-tight wrap. When the tow expands, after release of the pressure on the tow, the tow springs back, somewhat, creating a vacuum within the air-tight wrap. The internal pressure created by the vacuum is enough to keep the sides substantially flat.
In US Patent Publication No. 2004/0159658, a bale with substantially flat sides (i.e., the top and bottom) is made by a technique in which the compressed tow is packaged in an air-tight wrap and then a vacuum (i.e., from an external source) is drawn within the wrapped tow.
In US Patent Publication No. 2006/0243142, a bale with substantially flat sides (i.e., top and bottom) is made by a technique where the tow is compressed between two protruding surfaces. These protruding surfaces may be convex, ellipsoidal, spherical, polyhedral (i.e., tetrahedral or pyramidal) or have curved or straight linear profiles. US Patent Publication No. 2006/0243142 at Paragraph [0036] and FIGS. 1 and 4. This technique uses long ‘press cycles’ of 10 and 20 minutes. Ibid., Table 1.
In US Patent Publication No. 2006/0249406, a bale with substantially flat sides (i.e., the top and bottom) is made by a technique where the tow is compressed between two platens with convex surfaces. The convex surfaces are a smooth surface, e.g., continuous, faceted, or stepped. US Patent Publication No. 2006/0249406, Paragraph [0022] and FIGS. 2 and 3. This technique uses compression period of about 1 second to several minutes. Ibid., Paragraph [0023].
There is a need for a method to make bales of cellulose acetate tow that have substantially flat sides (e.g., the top and bottom) and which the tow may be easily de-baled.